The present disclosure relates to an image forming apparatus including a developing unit which develops an electrostatic latent image formed by an electrophotographic process.
In an image forming apparatus using the electrophotographic process such as a copying machine, a printer, facsimile, or the like, toner is supplied to an electrostatic latent image formed on the surface of an image bearing member such as a photosensitive drum to develop (visualize) the electrostatic latent image in the form of a toner image and the toner image is transferred to and fixed on a recording medium.
In such an image forming apparatus, when the toner image is formed on the surface of a photosensitive drum, toner is consumed. Thus, to keep the toner density (ratio of toner with respect to carrier) of developer stored in the developing unit on a constant level, the developing unit is supplied with toner from a toner cartridge. However, when images having a high coverage rate are printed continuously, the density of toner stored in the developing unit decreases extremely, so that formed images may be weak or image unevenness may occur in the images.
When the toner density drops below a control standard value (hereinafter also referred to as “toner density control standard value”) corresponding to the coverage rate of images, printing is stopped temporarily and the toner is supplied while developer in the developing unit is agitated. Then, when the toner density in the developing unit is restored to the toner density control standard value, printing is restarted. Such an image forming apparatus has been proposed.
Further, a technology of inhibiting a drop in the toner density in the developing unit has been proposed by changing the toner density control standard value when printing images having a high coverage rate are printed continuously.
However, the former technology has a possibility that when images are printed continuously regardless of the coverage rate, supply of toner to meet consumption of toner may be delayed. Further, according to the former technology, when an image having a high coverage rate is changed to an image having a low coverage rate, there is a possibility that print action may be stopped to execute supply of toner although the print action does not have to be stopped.
Even for images having a high coverage rate, it is not always necessary to stop the print action and supply toner if print speed is slow. However, the former technology has a possibility that even in this case, the print action may be stopped to supply toner.
On the other hand, according to the latter technology, once the toner density control standard value is raised due to continuous printing of images having a high coverage rate, the toner density may be raised too much when images having an ordinary coverage rate are printed. Thus, there is a possibility that fogging on a white background (adhesion of improperly charged toner) may be generated.